Flexible unbonded pipes of the present type are for example described in the standard “Recommended Practice for Flexible Pipe”, ANSI/API 17 B, fourth Edition, July 2008, and the standard “Specification for Unbonded Flexible Pipe”, ANSI/API 17J, Third edition, July 2008. Such pipes usually comprise an inner liner also often called an inner sealing sheath or an inner sheath, which is the innermost sealing sheath and which forms a barrier against the outflow of the fluid which is conveyed in the bore of the pipe, and one or more armoring layers. Often the pipe further comprises an outer protection layer which provides mechanical protection of the armor layers. The outer protection layer may be a sealing layer sealing against ingress of sea water. In certain unbonded flexible pipes one or more intermediate sealing layers are arranged between armor layers.
In general flexible pipes are expected to have a lifetime of 20 years in operation.
Examples of unbonded flexible pipes are e.g. disclosed in U.S. Pat. No. 6,978,806; U.S. Pat. No. 7,124,780; U.S. Pat. No. 6,769,454 and U.S. Pat. No. 6,363,974.
The term “unbonded” means in this text that at least two of the layers including the armoring layers and polymer layers are not bonded to each other. In practice the known pipe normally comprises at least two armoring layers located outside the inner sealing sheath and optionally an armor structure located inside the inner sealing sheath normally referred to as a carcass.
The term “sealing sheath” is herein used to designate a liquid impermeable layer, normally comprising or consisting of polymer. The term “inner sealing sheath” designates the innermost sealing sheath. The term “intermediate sealing sheath” means a sealing sheath which is not the inner sealing sheath and which comprises at least one additional layer on its outer side. The term “outer sealing sheath” means the outermost sealing sheath.
The armoring layers usually comprise or consist of one or more helically wound elongated armoring elements, where the individual armor layers are not bonded to each other directly or indirectly via other layers along the pipe. Thereby the pipe becomes bendable and sufficiently flexible to roll up for transportation.
For many applications a pipe of the above type will need to fulfill a number of requirements. First of all the pipe should have a high mechanical strength to withstand the forces it will be subjected to during transportation, deployment and operation. The internal pressure (from inside of the pipe and outwards) and the external pressure (from outside of the pipe) are usually very high and may vary. The unbonded flexible pipes are therefore usually armored with at least one pressure armor layer composed of helically wound steel elements which are wound with a relatively high winding angle to the axis of the pipe. It is well known that such pressure armor steel elements require protection from contact with seawater and therefore the pressure armor steel elements are usually applied in an annulus between the inner sealing sheath and a sealing sheath most often together with one or more tensile armor layers comprising elongate armor elements wound with a relatively low winding angle to the axis of the pipe.
In order to reduce the risk of damaging the pipe due to increased pressure in the annulus caused by gasses, such as CO2, H2S and H2O, which diffuse through the inner sealing sheath from the bore of the pipe, the annulus often comprises a means for venting such annulus normally through passages between the tensile armor elements. Another purpose of draining gasses diffused through the inner sealing sheath is that such gas often comprises highly corrosive components originating from the fluid transported in the bore of the pipe, such as H2S.
However, in some situations the pressure armor layer or layers and the tensile armor layer or layers are separated by a sealing sheath and in such cases it has shown to be difficult to provide a sufficient and safe draining of undesired gas from the annulus comprising the pressure layer(s), in particular because such pressure armor elements generally need to be very closely packed and are often interlocked in order to provide a sufficient protection against burst and damage due to high internal pressure in the pipe.
U.S. Pat. No. 7,124,780 describes a flexible tubular pipe having from internal and out an inner sealing sheath, a pressure armor, an intermediate sealing sheath, at least one tensile armor layer and an outer sealing sheath. The annulus formed by the inner annular space between the inner sealing sheath and the intermediate sealing sheath comprises a drainage layer which is used to drain the gases present in the annular space, wherein the drainage layer is formed by a short-pitch winding of at least one long element having transverse drain holes or spaces which can drain gases between the successive turns of the winding.
U.S. Pat. No. 6,769,454 describes a flexible pipe comprising an inner sealing sheath defining an inner longitudinal passage and at least one longitudinally extending tube embedded in the inner sealing sheath and extending for the length of the inner sealing sheath. The tube is adapted to receive any gases permeating through or into the inner layer and to vent gases from the pipe. The pipe further comprises an outer sealing sheath extending over the inner sealing sheath.
The above solutions may help in removing undesired gasses from the annulus. However, the solution provided simultaneously results in an undesired weakening of the pipe. Replacing parts of the armor layer with drainage means inevitably reduces the strength of the pipe. Further it should be mentioned that the intactness of the inner sealing sheath is very important for the strength of the pipe against leaks.